Not all is as it seems in the universe. Only 5% of the universe is made up of normal matter we can observe. Dark energy and dark matter are more than astronomy terms — they may hold the key to unlocking the mysteries of the cosmos.

Astronomy Terms: What Is Dark Energy?

We know how much dark energy the universe holds because we can observe how it affects the universe’s expansion. Approximately 68% of the universe consists of dark energy. Dark matter accounts for another 27%.

This leaves about 5% for everything that we can observe in the universe — everything on earth and everything we can see in the visible universe. This 5% of “normal space” is perhaps misnamed.  

• There are many theories regarding what dark energy is. It could be a property of space. It’s one of the hardest astronomy terms to define. Albert Einstein was the first scientist to figure out that empty space is more than nothingness. It's actually possible for more space to come into existence. Part of Einstein's gravity theory predicts that empty space has its own energy.
• Another theory of how space can acquire new energy derives from quantum physics. In this approach, empty space has temporary particles that continuously form and disappear. However, physicists were unable to reconcile how much energy would exist in space using this model.
• Another explanation scientists have put forth involves a new kind of energy fluid or field. This substance would have to fill all of space, and its effect on the expanding universe counteracts that of matter and normal energy. The working name for this substance is quintessential. If it exists, we don't know its properties or why it exists. So, the mystery remains.

In short, dark energy could be a property of space, a new theory of gravity, or a dynamic fluid in space. Better data is needed to solidify each of these theories.

Astronomy Terms: What Is Dark Matter?

Dark matter and dark energy are related astronomy terms. By pairing a theory regarding the composition of the universe with cosmological observations, scientists have calculated the amount of dark matter as 27%. But what dark matter is, is still a mystery.

• First, the term dark means that it does not consist of stars and planets like the bodies we see in the night sky. Scientists know that there's too little visible matter to make up the 27% required by observations.
• Second, it differs from normal matter, which is made up of particles called baryons. If this were the case, we would be able to detect baryonic clouds by radiation absorption measurements.
• Third, dark matter differs from antimatter. Antimatter contains gamma rays produced when it annihilates upon contact with matter.
• Finally, no galaxy-sized black holes exist, based on the number of gravitational lenses observed. Any high concentration of matter bends light from far away objects that pass. And there aren’t enough of these occurrences to make up dark matter's 27% contribution.

One possible suggestion is baryonic matter, which might make up dark matter if it's concentrated in brown dwarfs or chunks of heavy elements. Another solution is an unknown matter.

The Mystery is Yet to be Solved

Understanding astronomy terms can help you learn what questions to ask about the universe, but the most fundamental questions — the universe's origin and eventual fate are still up in space. (For more space info, please click here.)